The heterodyne concept is employed extensively in radio receivers and transmitters. The principle is based on shifting a receive signal to a much lower fixed intermediate frequency (IF). During this process the information modulated on the carrier remains unaltered and occupies an equal bandwidth centered on the intermediate frequency. The lower frequency domain allows simpler signal processing. The required frequency translation to the intermediate frequency is accomplished by mixing the incoming signal with a locally generated signal which differs from the incoming carrier by the intermediate frequency. In the typical heterodyne receiver, one oscillator-mixer pair is used to shift the receive or transmit frequency. However, a receive signal may be shifted two or more times to realize various analog processing advantages such as filtering, image rejection and demodulation. The same applies to a transmitter operating in the reverse direction.
In a time division multiple access system, such as a personal communication system, many modulated carriers are spaced several channels apart. Typically, a fixed IF oscillator and a stepped RF local oscillator (LO) are employed to allow channel selection. The LO is set to one IF frequency below or above the desired RF channel and tracks this channel.
In the specific case of the North American PCS frequency allocation, FCC standard Part 24 (24.229) specifies that the frequency range 1850.01-1909.95 MHz is available for base-station reception/mobile transmission and 1930.05-1989.99 MHz is available for base-station transmission/mobile reception, yielding a maximum bandwidth of 59.94 MHz each for reception and transmission. Within each of these ranges there are individual bands, A through to F; these bands are further divided into individual channels of 30 KHz bandwidth. According to the frequency allocation a mobile transceiver operating in Band A, for example, receives its signal in the appropriate channel of receive Band A, a frequency 80.04 MHz higher than the signal it transmits; likewise for bands B through to F.
A microcell is a basestation for an indoor wireless communication system. Microcells operate according to the frequency allocation standard referred to above and are capable of simultaneous transmission and reception of signals. Usually a microcell is provided with an external duplex filter which determines the band that the microcell operates in. However, there could be a need for a microcell which is capable of operating over all of the bands A to F. Such a device would enable a single product to be manufactured for use in different bands rather than require different devices for the different bands, resulting possibly in economies of scale particularly with greater and greater integration of components within the device.
A problem in designing a single basestation like a microcell in this way is that there is only a narrow (20.10 MHz) isolation band between the uppermost microcell receive frequency, 1909.95 MHz, and the lowermost microcell transmit frequency, 1930.05 MHz as opposed to about 65 MHz for a device designed to operate only on a single band. Consequently, achieving adequate filtering for an all-band microcell is problematic since it is difficult to design a low cost microcell comprising band-pass receive and transmit filters meeting the required specification, namely; a centre frequency around 1900 MHz, a bandwidth of 59.94 MHz and a desirable 100 dB rejection beyond 20 MHz of the filter band-edges.